APPLICATION HINTS

Time Reading Using Interrupt

In systems such as point of sale terminals and data loggers

time reading is usually only required on a random demand

basis Using the data-changed flag as outlined in the section

on methods of operation is ideal for this type of system

Some systems however need to sense a change in real

time eg

industrial timersprocess controllers TVVCR

clocks any system where real time is displayed

The interrupt timer on the MM58274C can generate inter-

rupts synchronously with the time registers changing using

software to provide the initial synchronization

In single interrupt mode the processor is responsible for ini-

tiating each timing cycle and the timed period is accurate to

g

1 ms

In repeated interrupt mode the period from the initial proces-

The following interrupts maintain accurate delay periods rel-

ative to the first timeout Thus to utilize interrupt to control

time reading we will use repeated interrupt mode

In repeated mode the time period between interrupts is ex-

act which means that timeouts will always occur at the

same point relative to the internal clock setting pulses The

case for 01s interrupts is shown in

Figure A-1 The same is

true for other delay periods only there will be more clock

setting pulses between each interrupt timeout If we set up

the interrupt timer so that interrupt always times out just

after the clock setting pulse occurs (

Figure A-2 ) then there

is no need to test the data-changed flag as we know that

the time data has just changed and will not alter again for

another 100 ms

This can be achieved as outlined below

1) Follow steps 1 and 2 of the section on interrupt program-

ming In step 2 set up for repeated interrupt

2) Read control register AD0

This is a dummy read to reset

the data-changed flag

3) Read control register AD0 until data-changed flag is set

4) Write 0 or 2 to control register Interrupt timing com-

mences

Time Reading with Very Slow Read Cycles

If a system takes longer than 100 ms to complete reading of

all the necessary time registers (eg when CMOS proces-

sors are used) or where high level interpreted language rou-

tines are used then the data-changed flag will always be set

when tested and is of no value In this case the time regis-

ters themselves must be tested to ensure data accuracy

The technique below will detect both time changing

be-

tween read strobes (ie between reading tens of minutes

and units of hours) and also time changing

during read

which can produce invalid data

1) Read and store the value of the

lowest order time register

required

2) Read out all the time registers required The registers

may be read out in any order simplifying software require-

ments

3) Read the lowest order register and compare it with the

value stored previously in step 1 If it is still the same then

all time data is good If it has changed then store the new

value and go back to step 2

In general the rule is that the first and last reads

must both

be of the lowest order time register These two values can

then be compared to ensure that no change has occurred

This technique works because for any higher order time reg-

ister to change all the lower order registers must also

change If the lowest order register does not change then

no higher order register has changed either

TLF11219 – 12

FIGURE A-1 Time Delay from Clock Setting Pulses to Interrupt is Constant

TLF11219 – 13

FIGURE A-2 Interrupt Timer Synchronized with Clock Setting Pulses

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